1. Field of the Invention
This invention relates generally to an electrophotographic-type imaging station, and more particularly, to an electrophotographic-type imaging station capable of producing an informational array pattern of enhanced resolution on a photosensitive medium.
2. Background Information
Electrophotographic-type imaging stations or printers operable to record information such as images or indicia on a photosensitive medium are well known. These imaging stations convert electrical signals to beams of light energy of varying intensities which are projected onto the surface of the photosensitive medium. The projected beams form an informational array pattern defined by the beams on the medium surface. Depending upon the type of photosensitive medium utilized, the informational array pattern representative of the image or indicia is either permanently fixed on the photosensitive medium through a photodevelopment process or transferred onto a recording medium which is brought into contact with the informational array pattern within the imaging station.
For example, one type of electrophotographic imaging station or printer utilizes a photosensitive medium in the form of a photoreceptor which is coated on a rotating drum designed to accept and hold a uniform electrostatic charge. After the uniformly charged surface of the photoreceptor is illuminated by light from an optical source to form an informational array pattern thereon which defines an image or indicia, the array pattern is developed by attracting toner particles. The amount of toner attraction varies with the intensity of the light projected from the optical source, and the array pattern is formed according to the differences in the toner density. Printing is completed by transferring this array pattern onto a recording medium such as paper by bringing the paper into contact with the photoreceptor.
Other known imaging stations or printers are operable to permanently record images or indicia directly on the surface of the photosensitive medium. These devices utilize a photosensitive medium in the form of an image-recording sheet of photosensitive paper which is passed in proximity to a light source. The surface of the paper is illuminated by beams of light energy projected from the source to form an informational array pattern which defines the image or indicia to be recorded. The beams of light energy develop the photosensitive paper to form the desired image or indicia.
With each of the imaging stations or printers described above, a light source in the form or either a laser scanner, a light-emitting diode array or a combination light source/liquid crystal shutter cell assembly is generally utilized. Both the diode array and the light source/liquid crystal shutter cell assembly project a linear array of beams of light energy towards the surface of the moving photosensitive medium so that recording on the medium is done in row-at-a-time fashion. With the laser scanner, a single beam of light energy projected towards the moving medium is reflected by a rotating prism-type assembly so that recording on the medium is done in dot-at-a-time, row-at-a-time fashion. Since each of the above-mentioned light sources projects either an array of generally roundish or square beams of light energy to form each projected row, or a generally roundish dot of light energy in the case of a laser scanner, the resolution of the projected informational array pattern taken in a direction perpendicular to each projected row is the same as the resolution of each row. Thus, the resolution of the informational array pattern is symmetrical in that the resolution of the pattern is the same in the directions both parallel with and perpendicular to the direction of movement of the photosensitive medium.
Since the resolution of the informational array pattern projected by conventional electrophotographic-type imaging stations or printers is symmetrical, it is extremely difficult to vary the gray scale of the array pattern without incurring a loss of array pattern resolution. With the imaging stations or printers of the prior art, an array pattern of desired gray scale is formed by dividing the pattern into a plurality of picture elements (PEL) each containing several individually addressable locations, and selecting the number of "on" or actually addressed locations in each PEL. For example, an informational array pattern having a resolution of three hundred (300) dots per inch (dpi) in directions both parallel with and perpendicular to the direction of movement of the photosensitive medium may be divided into a plurality of 4.times.4 PELs each containing sixteen individually addressable locations. By varying the total number of individually addressable locations in each PEL actually addressed, the gray scale of the informational array pattern may be varied incrementally from between 0 and 100%. However, by forming each PEL in 4.times.4 matrix form, the resolution of the array pattern in directions both parallel with and perpendicular to the direction of movement of the photosensitive medium is reduced from 300 dpi to 300/4=75 dpi.
The loss of resolution to achieve desired gray scale is unacceptable in many printing applications, and therefore there is a need for an improved electrophotographic-type imaging station or printer capable of projecting an informational array pattern onto the surface of a photosensitive medium which overcomes these shortcomings. Specifically, the improved imaging station or printer must be capable of projecting an array pattern having a resolution in a direction parallel with the direction of photosensitive medium movement which is greater than the resolution of the array pattern in a direction perpendicular to the direction of movement of the medium. Projecting an informational array pattern in this manner will permit a desired pattern gray scale to be achieved without a loss in the original resolution of the pattern taken in the direction perpendicular to the direction of medium movement.